The Fourth Ecumenical Council (Chalcedon, 451)

Eutyches, an archimandrite in Constantinople, taught that after the union only one nature, the divine, remained in Christ. A council at Ephesus in 449 acquitted him; in the West it was called the “Robber Council”. The Emperor Marcian, with the Empress Pulcheria, convened a new council at Chalcedon, across from Constantinople, in 451.

The Council, traditionally of 630 fathers (historians give some 500–600 participants), received the Tome of Pope Leo the Great and issued a definition of faith: the Lord Jesus Christ is one person (hypostasis) made known in two natures, divine and human, “without confusion, without change, without division, without separation”. Thirty canons were adopted; the 28th, on the rights of the see of Constantinople, was not accepted by Rome.

The decisions were not accepted by the Churches of Egypt, part of Syria, Armenia and later Ethiopia — today's Oriental (non-Chalcedonian) Orthodox Churches. They do not regard themselves as followers of Eutyches and confess the one incarnate nature of God the Word in the spirit of St Cyril. Theological dialogue with these Churches continues today.

The Chalcedonian definition became the foundation of Orthodox Christology, developed further at the Fifth and Sixth Ecumenical Councils.